I can't make a definitive claim in the definition of "alt-history," but, I agree with JA in that, any alternative has to turn on an event which changes the original outcome (which, if I remember right, is also called a "counterfactual?") Cor instance, the "What if...?" series posited 13 ways The U.S. Could have lost the Revolutionary War (quite a few of of which involved Washington getting captured or killed, or some other such thing). They present the situation, then posit the effect, with more than one example focusing on someone being removed from the situation (EX:the anecdote about Churchill nearly being run over by a car [if I remember right]). I would think an alternative history would have to turn on some event for which a substantially different outcome has a "plausible" cause (City on the Edge of Forever?).
Historical fiction, on the other hand, doesn't present alternative outcomes, but rather places fictional characters in the actuality, or crafts story using real folks, positing what they might have said or thought, intertwined with the facts (a la "Killer Angels").
So, if you place fictional characters in a real historical setting, actual historical events occur (are simulated), and their effect on major events is non-existent, then you have historical fiction. If the setting is accurate, but not really geared toward events that make a difference, then, to me, that's just plain fiction. It's the difference between characters fighting in the Battle of Gettysburg, participating in Chamberlain's desperate bayonet charge, and characters fighting in some location in the wilderness, with the war as setting alone, and the battle not making a difference.
But, really, this is just splitting hairs. I don't think most players give a shit whether they're in the actual El Dorado, or whether Dark Albion is 100% accurate. All that is secondary to the fun.
Or should be, anyway...